1. Field of the Invention
The invention relates to a method of machining a work piece with a focused particle beam to form a sample, comprising:                Inserting the work piece in a particle-optical apparatus,        Machining the work piece to form the sample by exposing the work piece to a focused particle beam,        Removing the sample from the particle-optical apparatus,        
2. Description of Related Art
For the inspection of samples, e.g. samples taken from a semiconductor wafer, in a Transmission Electron Microscope (TEM) samples with a thickness of less than 100 nm and preferably less than 50 nm, are routinely used.
U.S. Pat. No. 5,270,552 discloses forming samples from a work piece such as a semiconductor wafer by first inserting the wafer in a Focused Ion Beam apparatus (FIB). A wedge or lamella of material is formed by milling a trench with a focused ion beam and then separating the wedge or lamella from the work piece. This wedge or lamella, with a thickness in excess of the desired thickness of e.g. 50 nm, is attached to a manipulator prior to the separation with e.g. ion beam induced deposition (IBID). A part of the wedge or lamella is then machined to a membrane with the desired thickness by milling with the focused ion beam, thereby forming the sample.
Although not disclosed in said U.S. patent, it is well known to the person skilled in the art to transport the thus formed sample, attached to the manipulator, to a TEM grid, attach the sample to the TEM grid and separate the sample from the manipulator by cutting it free from the manipulator with the ion beam. The sample, mounted on the TEM grid, is then taken from the ion beam instrument and transferred to a TEM for inspection.
The method is also known from U.S. Pat. No. 7,002,152. This patent discloses forming a sample by alternating exposure of opposite sides of a sample to a glancing ion beam. Here the work piece is not a wafer, but a lamella to be thinned. The use of a glancing ion beam minimizes e.g. crystallographic damage and the implantation of ions from the ion beam into the sample.
It is noted that the work piece may comprise or consist of semiconductor material, as is the case when machining a semiconductor wafer, but may also comprise or consist of other material, such as organic tissue in the form of a bacterium or a cell, a pharmaceutical composition, a polymer or a metallic sample.
A disadvantage of the known methods is that the finished sample, when transferring it from the FIB to the TEM, is exposed to air. The exposure to air may results in chemical changes of the surface of the sample, such as immediate oxidation. Such chemical changes have a negative impact on the quality of the samples, and may even render the sample unfit for subsequent inspection. In this context the quality of the sample is a measure of the condition of the sample during inspection to provide information of the sample in pristine condition (or to restore said information) when inspecting the sample.
A similar deterioration of the sample occurs when the sample is stored for a prolonged period for e.g. future investigation or comparison.
It is even known that a similar deterioration may occur when exposing the sample to the low pressures used in such a FIB for a prolonged time, as some residual oxygen or water is present, causing a slow oxidation of the sample.
A known partial solution to the problem is to transport and/or store the sample in an inert environment, such as in an evacuated transport unit or in a transport unit filled with an inert gas.
A problem to this partial solution is that even at a low (partial) pressure of e.g. oxygen or water oxidation may occur, and thereby the storage or transport of samples by this method does not reliably result in high quality samples. Also the costs associated with this partial solution, including the addition of a vacuum tight interface between the transport unit and the FIB and/or TEM, results in complex and expensive adaptations of the instruments used.
There is a need for a sample preparation method in which the sample is not deteriorated when taken from the particle-optical apparatus used for machining the work piece.